Semiconductor lithography utilizing optical imaging systems has been carried out for many years. The process involves the creation of relief image patterns through the projection of radiation within or near the UV visible portion of the electromagnetic spectrum. Earlier methods of optical semiconductor lithography utilized a proximity printing technique, where a photomask with the desired device pattern image was held close to the surface of a photosensitized silicon wafer surface, transferring the image the image to the surface. Resolution, device size, and device yield are limited using this approach because of the lack of reduction optics. Modern reduction projection techniques using step-and-repeat or step-and-scan optical systems minimize some of the problems encountered with earlier proximity lithography methods and have lead to the development of tools that currently allow resolution below 0.15 μm.
Semiconductor device features are generally on the order of the wavelength of the ultraviolet (UV) radiation used to pattern them. Currently, exposure wavelengths are on the order of 150 to 450 nm and more specifically 157 nm, 293 nm, 248 nm, 365 nm, and 436 nm. The most challenging lithographic features are those which fall near or below sizes corresponding to 0.5λ/NA, where λ is the exposing wavelength and NA is the objective lens numerical aperture of the exposure tool. As an example, for a 193 nm-wavelength exposure system incorporating a 0.65NA objective lens, the imaging of features at or below 0.13 micrometers is considered state of the art. Generally, systems employ Köhler type illumination and an effective source that is shaped circularly. More recently, source shapes have been varied from this conventional circular shape to best optimize illumination conditions for a specific photomask pattern, wavelength, NA, and other imaging parameters. Off axis illumination using dipole illumination, with a pair or circular source shapes oriented in the direction of mask geometry can offer a significant enhancement to imaging performance. This is because only oblique illumination at an optimized illumination angle can be designed to allow projection of a single orientation of mask diffraction energy at the outermost edges of an objective lens pupil. The problem with dipole illumination arises when geometry of both X and Y (or horizontal and vertical) nature is considered since imaging is limited to features oriented along one direction in an X–Y plane. Additionally, the use of circular pole shapes could be improved by using poles with square or rectangular shaping. Four pole or quadrupole source shapes are an example of a modification for X and Y oriented geometry [see for instance U.S. Pat. No. 5,305,054]. Here, four circular poles are utilized to accommodate the mask geometry located along two orthogonal axes. The use of multiple circular shaped poles is not the best shaping for use with mask geometry oriented on orthogonal X and Y-axes however. I have discovered that particular square pole shapes extended along X and Y axes, forming chevron shapes at the corners of an illumination source, show superior performance to other illumination approaches.